1. Field of the Invention
The present invention relates to a run-flat tire of wherein the tread portion is prevented from buckling to improve run-flat performances.
2. Description of Related Art
In general, as shown in FIG. 5, run-flat tires have a great mass of rubber (b) along the inside of the sidewall portions (a) to support tire loads during running under flat conditions. Thus, an increase in the tire weight is unavoidable. Further, due to the heat generated during running under flat conditions as well as normal conditions especially at a high speed, the internal temperature is liable to become very high, which sometimes decreases the durability of the sidewall portions contrary to expectation. Therefore, it is preferable to decrease the rubber volume in the sidewall portions as far as possible. However, if the rubber volume is decreased, the bending deformation of the sidewall portion (a) becomes relatively large as shown in FIG. 5. Accordingly, the heat generation increases and the durability decreases again. Thus, it is difficult to improve run-flat performances and durability at the same time.
In order to improve the run-flat performances and durability, the inventors studied and discovered that when the tire air pressure is zero the bending deformation (ya) of the sidewall portion (a) has a close relation with the buckling deformation (yt) of the tread portion (t). FIG. 4 shows such a relationship, wherein the bending deformation (ya) of the sidewall portion is indicated by a clearance (ha) of the inner surface of the tire measured along the radial direction at a distance (f) of 10 mm axially inwardly from the edge (e) of the ground contacting area as shown in FIG. 5. Thus, the smaller the clearance, the larger the bending deformation. The buckling deformation (yt) of the tread portion is indicated by the amount of lift-up (ht) measured at the tire equator from the road surface to the inner surface of the tire. Thus, the larger the lift-up, the larger the buckling deformation.
As shown in FIG. 4, the bending deformation of the sidewall portion was found to increase as the buckling deformation of the tread portion increases. Therefore, if the buckling deformation is decreased, the bending deformation can be decreased.
Further, it was also discovered that it is not effective for controlling the buckling deformation to dispose a reinforcing cord layer in the tread portion. The reinforcing cord layers such as belt, carcass and the like do not show their rigidity until the tire is inflated to a normal pressure under which a full tension is applied to the cords. Thus, it is difficult to increase the resistance to buckling deformation under flat conditions.